Hello Nicolas,
To clarify a point from your initial question:
K-series hardware will indeed support SFPs in any SFP+ port.
There are a few caveats about 'approved vendor' but I'll go out on a limb and say most SFPs should work. Why the conditional 'approved' support? If we've vetted the optics in our labs there's additional confidence in (and support for) interoperability.
Best,
Mike